The present invention relates to a fuel supply system for a multi-cylinder internal combustion engine, and more particularly to a fuel supply system designed so as to inject a fuel into a manifold entrance. In a spark ignition-type multi-cylinder internal combustion engine, a SPI method (single point injection method) is employed wherein fuel injection is effected with a single fuel injection valve provided in a manifold entrance. The SPI method makes it possible to remarkably simplify the structure of the fuel supply system and reduce the cost thereof, in comparison with a method in which fuel injection is effected with a plurality of fuel injection valves each provided in the intake port of each cylinder, respectively.
However, the SPI method has drawbacks that it is difficult to obtain satisfactory results in a fuel atomizing characteristic and a mixture distribution characteristic.
To eliminate these drawbacks, an attempt is made to cause the fuel being injected from the fuel injection valve to collide against a diffusion plate, thereby effecting the atomization of the fuel. Another attempt is to effect high-speed fuel injection directed to the clearance between the throttle valve and the bore of the manifold in which the throttle valve is rotatably provided, thereby effecting a fuel atomization by the guideline of a high-speed current of air.
However, with these attempts, the fuel partially sticks to the passage wall surface, so that supply responsibility is apt to be lowered.
Moreover, another method is proposed which injects a fuel onto a riser floor subject to exhaust heat, in place of injecting the fuel onto the inner wall surface of the intake passage, thereby vapourizing the fuel.
However, with this method, the fuel flow has a tendency to be influenced by the amount of the intake air. As a result, according to driving conditions, it is likely that the distribution of the mixture becomes unsatisfactory.
A still further method is proposed which introduces air through a bypass passage whose inlet portion is provided around the nozzle of a fuel injection valve, while whose outlet portion is provided in the downstream side of the throttle valve, thereby effecting the fuel atomization. Although this method is effective at the time of extremely low load operation, the atomizing effect is rapidly reduced in the region of medium and high loads.